Chapter 13 Outline - Chapter 13 The Viruses and Virus-Like...

Chapter 13 The Viruses and Virus-Like Agents 13.1 Foundations of Virology • Many Scientists Contributed to the Early Understanding of Viruses • Dimitri Ivanowsky and Martinus Beiherinck studied the tobacco mosaic virus • Walter Reed studied foot-and-mouth disease and yellow fever • Frederick Twort and Felix d’Herelle studied bacteriophages • In the 1930s, it was discovered that viruses are nonliving agents composed of nucleic acid and protein • Alice M. Woodruff and Ernest W. Goodpasture developed a culture technique using chicken eggs 13.2 What Are Viruses? • Viruses Are Tiny Infectious Agents • Viruses are small, obligate intracellular parasites • They lack the machinery for generating energy and large molecules • They need a host eukaryote or prokaryote to replicate • The viral genome contains either DNA or RNA, but not both • The capsid is the protein coat, made up of capsomeres • The nucleocapsid is the capsid with its enclosed genome • Some capsid proteins are spikes that help the virus attach to and penetrate the host cell • Naked viruses are composed only of a nucleocapsid • Viruses surrounded by an envelope are enveloped viruses • A virion is a completely assembled, infectious virus outside its host cell • Viruses Are Grouped by Their Shape • Helical viruses have helical symmetry • Isocahedral viruses have isocahedral symmetry • Viruses that have both helical and isocahedral symmetry have complex symmetry • Viruses Have a Host Range and Tissue Specificity • A host range refers to what organisms the virus can infect • Host range depends on capsid structure • Many viruses infect certain cell or tissue types within the host (tissue tropism) 13.3 The Classification of Viruses • Nomenclature and Classification Do Not Use Conventional Taxonomic Groups • Viruses can be named according to a number of different conventions • The International Committee on Taxonomy of Viruses (ICTV) is developing a classification system • DNA viruses contain single- or double-stranded DNA genomes • RNA viruses contain single- or double-stranded RNA genomes • + strand RNA viruses have mRNA genomes • – strand RNA viruses have RNA strands that would be complementary to mRNA • Retroviruses are replicated indirectly through a DNA intermediate 13.4 Viral Replication and Its Control • The Replication of Bacteriophages Is a Five-Step Process • T-even group bacteriophages are virulent viruses that carry out a lytic cycle of infection • The phage nucleic acid contains only a few of the genes needed for viral synthesis and

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